Commercially available implantable medication delivery devices are exemplified by a Synchromed product marketed by Medtronic (of Minneapolis, Minn., USA) and a MIP product manufactured by Minimed, now a division of Medtronic. Both of these devices employ a medication reservoir comprising a bellows that contracts as medication is extracted by a pump mechanism. The reservoir volume change is accommodated by a second chamber which contains a propellant such as Freon in a gas/liquid equilibrium. The propellant functions to maintain a constant absolute pressure at body temperature. In the case of the MIP product, the propellant is a liquid at body temperature creating a negative pressure reservoir. In the case of the Syncromed product, the propellant is a gas at body temperature creating a positive pressure reservoir. In both cases, the medication reservoir is maintained at a constant absolute pressure by the propellant. Although the reservoir, and therefore the inlet side of the pump mechanism are at a constant absolute pressure, the tip of an output catheter and thus the outlet of the pump mechanism, are at ambient pressure. Ambient pressure typically varies as a function of environmental conditions including local barometric pressure and altitude, etc. In addition, variations in temperature can produce variations in reservoir pressure. The combined effect of these conditions can produce pressure differences in excess of 500 millibars across the pump mechanism. In order to seal and pump across a pressure difference of this magnitude, these exemplary systems require pumps of a size which are not well suited for implantation in space limited sites, e.g., the brain, eye, or ear.